Cardiovascular diseases are the leading causes of deaths of human being. Presently, urokinase and tissue-type plasminogen activator are used in clinic as major thrombolytic drugs, while heparin and hirudin are used as major anticoagulant drugs. Although thrombolytic therapy is successful in decreasing the death rate of patients suffering from thrombus, re-thrombus will often occur due to the entering of small gore into blood circulation. Therefore, heparin or hirudin is now used as anticoagulant agents in combination with thrombolytic agents for thrombolytic therapy.
However, thrombolytic agents and anticoagulant agents usually cause systematic haemolysis and anticoagulation due to their poor specificity, thereby leading to systematic haemorrhage.
In particular, tissue-type plasminogen activator (t-PA), streptokinase (SK), urokinase (UK) and urokinase-like plasminogen activator (u-PA) can activate the plasminogen, which in turn lyses clots at the site of thrombus. However, the activated plasminogen will also cause bleeding in non-thrombus locus. Staphylokinase (SAK) is a new plasminogen activator of native origin, and has certain specificity to the thrombus.
Hirudin (HV), which is a small protein, is regarded as the new anticoagulant agent due to its high affinity and selective inhibition to thrombin. However, hirudin is inclined to cause systematic haemorrhage in clinic. Moreover, there is no antagonist against hirudin hitherto.
Thus, to decrease side effects and increase therapeutic effects of thrombolytic drugs, it is important to increase their selectivity, targeting property and local concentration at thrombus clots, and to decrease drug concentration or activity at thrombus-free sites and the side effect of bleeding. The fusion protein in the prior art cannot reduce the side effect of bleeding.
Therefore, the development of anti-thrombus drug is focused on bifunctional medicament having both thrombolytic and anticoagulant activities. To combine thrombolytic activity with anticoagulant activity, the fusion of hirudin with SAK or SK have been studied. It showed that, if the N-terminal of hirudin was linked to C-terminal of SAK or SK, the hirudin will loss its anti-thrombin activity, while the plasminogen-activating activity of the thrombolytic protein is retained or partially retained; if the C-terminal of hirudin is linked to N-terminal of SAK, then due to the rapid degradation of N-terminal of SAK in vivo, the fusion protein will undergo degradation before functioning. Thus it is attracting in thrombolytic therapy to develop fusion protein having both thrombolytic and anticoagulant activities.